Examples of a technique relating to an injection molding process for foamed plastic include JP-A-62-246710, JP-A-4-214311, JP-A-7-9487, JP-A-8-300392, JP-A-10-156884, and JP-A-11-147235.
For example, JP-A-62-246710 discloses a known technique relating to an injection-foaming molding process for manufacturing a plastic formed article by foaming having an internal portion of a densely foamed structure. In this document, there is disclosed an injection-foaming molding process wherein an injection machine includes a mold for forming an article being connected to an injection port and being composed of a fixed mold and a movable mold movable toward or away from the fixed mold is used; and wherein a cavity being formed therebetween and being expanded or contracted by the movement of the movable mold is expanded to a predetermined space by retracting the movable mold before a foaming agent is injected into the mold.
JP-A-4-214311 discloses an injection-foaming molding process for manufacturing a plastic formed article by foaming having a densely foamed structure. In this process, a pair of mold members that can be fitted to each other and moved in the direction that the volume of a cavity is increased or decreased are each placed at corresponding positions for allowing the cavity to have a reduced volume; the mold is moved in the direction that the cavity volume is increased while a molten expandable resin is being injected into the cavity in such a manner that the pressure of the resin is maintained such that the resin does not foam; the mold is moved in the direction that the cavity volume is decreased, whereby the resin is compressed; a surface portion of the resin is cooled to solidify the surface portion; the pressure of the resin is reduced by moving the mold in the direction that the cavity volume is increased, whereby an inner portion of the resin is allowed to foam; and the resulting resin is cooled into an article, which is then demolded from the mold. An object of this process is to adjust the foaming magnification by controlling the clamping position of an injection-foaming forming machine.
JP-A-8-300392 discloses an injection-foaming molding process for manufacturing a plastic formed article by foaming having a foamed structure. In this process, it is stated that problems, such as a change in volume and a change in fluidity which are caused by foaming can be eliminated and the fluidity is therefore maintained satisfactorily with heating under control conditions a resin containing a foaming agent is heated in a cylinder at a temperature that is higher than or equal to the melting point of the resin but lower than or equal to the thermal decomposition temperature of the foaming agent. The injected resin is heated in, for example, a nozzle placed between the cylinder and a mold, whereby the foaming agent is heated to a temperature higher than or equal to the thermal decomposition temperature thereof. Therefore, the foaming agent is partly decomposed by heat depending on the kind of the foaming agent and starts to foam in a trace amount; however, almost of all the molten resin remains in a substantially unfoamed state. The resin is then injected into a cavity through a gate placed in the mold. Since the resulting resin is in contact with the mold or a thin skin material placed in the mold, the resin is quenched to a temperature less than or equal to the melting point thereof, whereby a surface portion of the resin that has not been foamed yet or has been slightly foamed is formed into a skin layer. In contrast, an inner portion of the resin that is not quenched in the cavity is allowed to foam and formed into a core. The skin layer is pressed against the inner wall of the mold or an inner surface of the skin material by the pressure of cells formed in the core; hence, the skin layer has no sink marks and any deterioration in quality such as distortion or a change in color do not occur on the surface of the skin layer because the foaming pressure is moderate. The formed article obtained is a laminate including the skin material of a high texture and the resin, securely bonded to the skin material, functioning as a substrate. The formed article includes the skin layer having no sink marks but high surface quality and strength and the core having been foamed and being therefore light in weight.
In other words, this document discloses an injection-foaming molding process in which a cavity of a mold is completely filled with an expandable resin in such a manner that the mold cavity is reduced in volume while the resin is being injected into the cavity or just after the injection is finished, the resulting resin is cooled until a surface portion of the resin in contact with a wall of the cavity is solidified to form a surface layer but an inner portion of the resin is maintained in a molten state, and the mold cavity is then expanded to a volume designed for an article to be formed.
However, in the technique dispersed in JP-A-62-246710, since the cavity volume is increased together with the start of the injection, cells present under a surface portion of the formed article are ruptured; hence, the surface (skin layer) does not have a non-cellular structure or a densely foamed structure with a lower expansion rate. Furthermore, since the cavity volume is large, the cavity cannot be completely filled with the resin when there is employed a mold having a narrow portion (a portion having a large flow resistance) at a remote portion from the gate.
On the other hand, in the process disclosed in JP-A-4-214311, in order to increase the pressure of the resin in the cavity to prevent the resin from foaming, the cavity volume is reduced at the start of the injection; hence, the flow resistance is large and the injection time is therefore long. Furthermore, after the surface portion of the formed article is cooled and solidified by expanding or contracting the cavity during the injection and the skin layer is thereby formed, the cavity is expanded again to allow the resin to foam. In this process, the control of foaming becomes difficult since the viscosity of the molten resin is increased due to a decrease in resin temperature, in addition to a prolonged cycle time for forming and the resulting lowered production efficiency.
In the process disclosed in JP-A-8-300392, after the cavity volume is reduced during the injection and this state is maintained for a predetermined period of time, the cavity is expanded, whereby the resin is allowed to foam. Therefore, no consideration has been taken as to the injection time, and the prevention of foaming at a reserving section located upstream of a screw together with the prolonged time of the forming cycle. Therefore, there is a problem in that cell nuclei formed in the resin-reserving section are not only mixed with an article to be formed in the next cycle but also foaming cannot be substantially controlled due to a increase in viscosity caused by a decrease in resin temperature when the injection time is prolonged.
On the other hand, examples of well-known processes for manufacturing a formed article by foaming include an injection-foaming molding process which is called as a short shot process in which a resin containing a foaming agent is injected into a mold in a short shot and then allowed to foam in the mold, or a full shot process in which a resin containing a foaming agent is injected into a mold in a full shot and then allowed to foam while the mold is being opened, whereby an article is formed.
The injection-foaming molding process wherein the resin is allowed to foam by increasing the cavity volume, which is represented by the full shot process, cannot be performed with a known, ordinary injection molding machine.
Therefore, in order to perform the injection-foaming molding process in which the resin is allowed to foam by increasing the cavity volume, an injection-foaming molding machine including a special clamping device must be used. As is known, injection-foaming molding machines having such a special function are disclosed in JP-A-48-650, JP-A-48-22164, and JP-A-48-56270.
The injection-foaming molding machines disclosed in those documents each include a clamping device, which is of a straight-hydraulic type. The clamping device is used to freely open a mold, placed between a fixed platen and a movable platen connected to a hydraulic cylinder, by moving the movable platen toward or away from the fixed platen. In each injection-foaming molding machine, the mold is opened or closed with the clamping device, whereby the cavity volume is increased and the resin is therefore allowed to foam.
However, in case of the injection-foaming molding process according to the above-mentioned conventional short shot process, there likely occurs such a phenomenon that the foaming agent contained in a molten resin is burst during injection of a molten resin containing a foaming agent into the cavity (this phenomenon is often called as a broken foam). In the case of a formed article containing broken foams, there is a problem that it has a surface roughness referred to as a swirl mark.
In contrast, in case of the injection-forming molding process according to the full shot process, a formed article by foaming thereby has substantially no surface roughness, that is, the formed article has a smooth surface. However, the full shot process, in which the resin is allowed to foam by increasing the cavity volume, has problems in that serious changes in foaming magnification, cell diameter, and/or skin layer thickness are caused in the foamed article, due to a quite minor deviation in the manner of opening the mold, for example, a deviation in timing for starting to open the mold, a deviation in opening speed, or a deviation in opening stroke. The hydraulic cylinder included in the clamping device, which is of the straight hydraulic type, has a problem in that operational reproducibility is low because the cylinder has operating characteristics sensitive to the temperature of oil used; hence, conforming products cannot be substantially manufactured continuously with high reproducibility.
Even if the position of the hydraulic cylinder is feedback-controlled to improve the operational reproducibility of the clamping device, the control accuracy does not substantially exceed the operational accuracy of the clamping device. Therefore, there are demands of developing a molding machine with which the full shot process can be performed with high accuracy and a method for controlling the machine, for performing the process, with high accuracy.
In the full shot process as well as ordinary injection molding processes, since a pressure is applied to the resin immediately after the completion of the injection in many cases, a portion of the resin must be allowed to remain at the front end of a screw placed in an injection cylinder. However, the resin portion remaining at the front end of the screw causes problems such as a decrease in foaming magnification, surface roughness, and defects in an article to be formed in the next shot because degassing or the formation of broken foams occurs in the resin portion due to the drop in the pressure in a charging operation for the next step.
Furthermore, the resin, which has been injected into the mold with a high pressure by the full shot process, has large pressure variations (differences in pressure) therein because there is a pressure gradient ranging from a gate section to a final injection point. The injected resin having such pressure variations has a difference in density between a portion close to the gate section and a portion away from the gate section. Therefore, there is a problem in that the formed article has a variation in the thickness of the skin layer and/or a difference in foaming magnification when the resin having such portions with different densities is allowed to foam as intact.